This invention relates generally to the construction of sewing machine needles of the type having a butt or clamping portion and an adjoining shank or shaft which terminates in a a point above which an eye is formed, and more particularly to a sewing machine needle of this type wherein a long, thread-guiding channel is formed in one side of the shaft which opens into the needle eye and which also has a cutout portion or chamfer formed in the shank above the eye on the side of the needle shaft opposite to the side in which the thread-guiding channel is formed. Sewing machine needles of this type are known and reference is made in this connection to U.S. Pat. No. 4,037,641.
Generally, a sewing machine needle functions as both a tool and as a thread-guiding element and must penetrate during its life a variety of materials to form millions of stitches transporting the needle thread to the thread loop catching device within a predetermined time and over a certain movement path, so that the thread can be gripped or caught to achieve the necessary interlacing and completion of the sewing cycles.
The materials sewn using such needles are in general conventional textile fabrics which may be textured, knitted materials from natural or synthetic fibers for clothing and lingerie, as well as leather, artificial leather or other synthetic laminated material used in the clothing and shoe industry. Moreover, other types of materials, such as paper, cardboard, synthetic foils, textures, meshes, and fleeces formed of glass, steel and asbestos fibers as well as a multitude of other materials in various combinations and designs can be sewn together by seams on sewing machines by means of needles and needle threads. The reliability and quality of the sewn seams are at least in part determined by the specific characteristics of the materials being sewn. In this connection, the elasticity and resistance to puncturing or penetration of the material being sewn often has a critical influence on the efficacy of the seam.
The most frequently used sewing machine needles for backstitching and chain-stitching machines generally include a shaft terminating at a point with an eye formed therethrough and in which a cutout or chamfer portion is formed above the eye to define an interspace between the needle shaft and a needle thread which runs in an upward direction parallel to the needle shaft, which interspace receives a thread catching device during operation of the machine. In conventional needle construction, depth of the chamfer portion is generally only about 25% of the nominal shaft diameter so that the interspace defined by the chamfer portion alone is not sufficient to assure a reliable penetration of the thread catching device. Moreover, the resistance to buckling of a sewing machine needle of this type is reduced by about 40% as compared to a needle which does not have a cutout or chamfer portion. Accordingly, in order to provide sufficient needle rigidity, especially in the case where the needle is used with a material having a high resistance to penetration, it is often necessary to use heavier needles than would otherwise be required by the thickness of the needle thread.
Conventional sewing needles often have an elongate thread-guiding channel formed therein which merge to the needle eye in which the needle thread runs during machine operation. However, due to production and technical reasons, the depth of the thread-guiding channel in the region directly above the needle eye is generally reduced so that the sewing thread is insufficiently protected at that region.
Moreover, in sewing machine needles which are currently formed using stamping techniques, the depth of chamfer portion is limited for technological reasons to a maximum of 30% of the nominal shaft diameter.
For the above reasons, a thread which is guided through a conventional sewing machine needle and which abuts the needle in a taut condition can be gripped or caught by the thread-catching device with only the greatest unreliability, especially when the needles are below Nm90. In other words, the interspace defined between the needle shaft and the needle thread by the cutout or chamfer portion is by itself insufficient to insure a reliable gripping by the thread-catching device. Accordingly, the space in which the thread-catching device is received must be enlarged through the formation of a loop which occurs during the lifting stroke of the needle. More particularly, after the needle has reached its lower dead center position it begins an upward stroke corresponding to a certain crank angle of the drive shaft. In this manner, a loop is formed at the needle whose size is determined at the angle through which the crank rotates during the upward stroke and thereby creating an interspace of a varying magnitude. For example, the interspace will remain relatively small if a highly elastic sewing thread is used in the sewing operation.
However, the angle through which the crank rotates during the loop formation step is relatively small and always occurs after the needle has reached the bottom dead center position of its stroke. These conditions impose severe limits on the construction and coupling of the mechanical functions of the machine which are determined by the kinematics of the sewing thread.
The production techniques presently utilized in the construction of conventional sewing machine needles in which cutout or chamfer portions are produced is constituted by a stamping process which necessarily results in the formation of burrs. Such techniques increase the number of necessary operations thereby increasing production costs as well as material losses whereby the quality of the needle is often insufficient.